Medical electrical lead including an inductance augmenter

ABSTRACT

A medical electrical lead includes an inductance augmenter assembly. The assembly includes an inductor coil formed of an insulated wire, which is wound about a non-conductive core and is electrically coupled in series between a conductor coil of the lead and an electrode of the lead.

TECHNICAL FIELD

The present invention pertains to medical electrical leads and moreparticularly to medical electrical leads including inductor elements.

BACKGROUND

Implanted medical devices often include elongate medical electricalleads carrying at least one conductor electrically coupled to atissue-contacting electrode; examples of such devices include, but arenot limited to, pacemakers, cardioverter-defibrillators andneurostimulators. It is known in the art that medical electrical leadscan act as antennas, which ‘pick up’ electromagnetic radiation ofradio-frequency (RF) pulses, for example used for Magnetic ResonanceImaging (MRI), and that these pulses can induce a appreciable amount ofcurrent in lead conductors. When this current is induced in a leadconductor, a relatively large current gradient can develop across theresistive interface between the electrode of that conductor and thetissue interfacing with the electrode; such a gradient causes heat to bereleased into the tissue, which may be injurious to the tissue.

It is further known in the art to insert, in series, an electromagnetictrap, in the form of an inductor element, between the conductor of alead and the tissue-contacting electrode. The inductor element acts as ahigh-frequency resistor attenuating RF pulse-induced current flowing tothe electrode and thereby reduces undesirable tissue heating. Althoughsuch inductor elements have been proposed for medical electrical leads,there is still a need for practical designs which integrate such anelement into medical leads without compromising other lead features, forexample, pertaining to ease of implantation, stable fixation and longterm performance of the lead.

Many medical electrical leads include elongate conductor coils coupledto tissue-contacting electrodes. In some cases, these conductor coilsmay be designed to have inductive properties that effectively reduce theundesirable tissue heating described above. However, in other cases, forexample, when the conductor coil must be stiff enough to transfer torquein order to steer a lead to, and/or fix a lead electrode at, a targetsite, it may not be possible for the coil to have the inductiveproperties that reduce tissue heating.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are not to scale (unless so stated) and are intended foruse in conjunction with the explanations in the following detaileddescription. Embodiments of the present invention will hereinafter bedescribed in conjunction with the appended drawings, wherein likenumerals denote like elements.

FIG. 1 is a schematic of an exemplary medical system including a medicalelectrical lead.

FIG. 2A is a plan view, with a partial section, of an electrodeassembly, according to one embodiment of the present invention.

FIG. 2B is a plan view, with partial sections, of a lead including theassembly of FIG. 2A, according to some embodiments of the presentinvention.

FIG. 3A is a plan view, with a partial section, of an electrodeassembly, according to another embodiment of the present invention.

FIG. 3B is a plan view, with partial sections, of a lead including theassembly of FIG. 3A, according to some embodiments of the presentinvention.

FIG. 4 is a section view through an inductor wire according to someembodiments of the present invention.

FIG. 5A is a plan view, with a partial section, of an inductanceaugmenter assembly, according to some embodiments of the presentinvention.

FIG. 5B is a perspective view of a component of the augmenter assemblyshown in FIG. 5A.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description providespractical illustrations for implementing exemplary embodiments of thepresent invention. Constructions, materials, dimensions, andmanufacturing processes suitable for making embodiments of the presentare known to those of skill in the field of the invention.

FIG. 1 is a schematic of an exemplary medical system 102 implanted in apatient 104. FIG. 1 illustrates medical system 102 including anelectronic medical device 106 contained in an hermetically sealedhousing 108 and coupled to a medical electrical lead 110. FIG. 1 furtherillustrates lead 110 including a tip electrode 122 and a more proximallylocated electrode 128, both positioned in a right ventricle of patient104; tip electrode 122 contacts tissue in the right ventricle, primarilyfor delivery of pacing stimulation from device 106. Electrode 128 mayform a bipolar pair with tip electrode 122, for pacing and sensing, maybe an independent defibrillation electrode, or may function both fordefibrillation and bipolar pacing and sensing. As previously described,lead 110 may act as an antenna picking up electromagnetic radiation ofradio-frequency (RF) pulses used for MRI. According to embodiments ofthe present invention, an electrical circuit of lead 110, which coupleselectrode tip 122 to device 106, includes an inductance augmenter, suchthat an inductance of the circuit is at least greater than approximatelyone micro Henry (μH), and, preferably, approaching approximately 4 μH.Inductances greater than approximately one μH have been shown to reducethe resultant current gradient across the electrode-tissue interface andthereby reduce undesirable tissue heating, but increasing the inductanceneeds to be balanced with circuit resistance, which typically should notexceed approximately 150 ohms for efficient pacing performance. FIG. 2Aillustrates one embodiment of such an electrical circuit or electrodeassembly.

FIG. 2A is a plan view, with a partial section, of an electrode assembly200, according to one embodiment of the present invention; and FIG. 2Bis a plan view, with partial sections, of lead 110 including assembly200, according to some embodiments of the present invention. FIG. 2Aillustrates assembly 200 including an inductance augmenter 210electrically coupled in series between an elongate conductor coil 201and electrode 122; electrode 122 is shown including an electrode surface222 in proximity to a distal tip thereof for tissue contact; a sharpeneddistal tip (not shown) and the helical form of electrode 122, well knownto those skilled in the art, facilitates fixation of electrode at animplant site via a screw-in method. According to preferred embodimentsof the present invention, conductor coil 201 is designed to efficientlytransfer torque from a proximal end to a distal end thereof in order toscrew in electrode 122; such a conductor coil may have a diameterbetween approximately 0.025 inch and approximately 0.035 inch and,although is shown herein formed of four wire filars, may have as few as2 filars and as many as six filars, each of which having a diameterbetween approximately 0.004 inch and approximately 0.007 inch. Becauseinductances of such coils are less than one μH, for lengths ranging fromapproximately 30 cm to approximately 70 cm, which are typical formedical electrical leads, an inductance augmenter, for example,inductance augmenter 210, is necessary. It should be noted thatconductor coils described herein may be formed from silver-cored ornon-silver-cored MP35N wire, or tantalum, known to those skilled in theart, and electrodes/electrode surfaces from platinum or platinum-iridiumalloy, also known to those skilled in the art.

FIG. 2B illustrates lead 110 including electrode assembly 200 as anextendable-retractable assembly, wherein the proximal end of conductorcoil 201 is coupled, within a connector 225, to a connector pin 205; inaddition to providing an electrical contact surface for coupling withdevice 106 (FIG. 1), pin 205 may be grasped in order to rotate coil 201and thereby screw electrode 122 into an implant site. Methods andarrangements of lead components suitable for connector 225 may be any ofthose known to those skilled in the art for extendable-retractable typeleads. FIG. 2B further illustrates a coaxial arrangement of conductorcoil 201 extending within an outer conductor coil 231, beingelectrically isolated from outer conductor coil 231 by a layer of innerinsulation 221. Outer conductor coil 231 is shown extending within alayer of outer insulation 241 to a distal end, which is mounted on, andcoupled to, an internal projection of electrode 128, for example, viacrimping or laser welding; an electrode surface 228 of electrode 128 isexposed external to the projection and has approximately the same outerdiameter as outer insulation 241. According to an exemplary embodiment,the outer diameter of outer insulation 241 and electrode surface 228 isabout 6 French or between approximately 0.07 inch and approximately0.085 inch. FIG. 2B further illustrates inner insulation 221 extendingover a proximal portion of inductance augmenter 210 and a distalinsulator member 245 extending over a distal portion of inductanceaugmenter 210 and having an outer diameter approximately equal to theouter diameter of electrode surface 228; inner insulation 221 andinsulator member 245 join together beneath electrode surface 228 tocomplete the electrical isolation of augmenter 210 from electrode 128.Those skilled in the art will appreciate that distal insulator member245 forms a sleeve head from which, and into which, electrode 122 may beextended and retracted, respectively, and includes a seal, for examplesealing member 215, to preventingress of bodily fluids. Those skilled inthe art will further appreciate that, in order to extend and retractelectrode 122, inductance augmenter 210 should be torsionally coupled tocoil 201 and to electrode 122. The term ‘torsionally coupled’ is usedherein to denote a mechanical coupling having the necessary rigidity totransfer torque, in a relatively efficient manner, and the durability tohandle the torsional loading. Although preferred embodiments of thepresent invention include torque transfer functionality, it should benoted that the scope of the invention is not so limited, and alternateembodiments of electrode assemblies include inductance augmenters thatneed not transfer torque.

Referring back to FIG. 2A, inductance augmenter 210 includes an inductorcoil 203 wound about a non-conductive core 206, which is coupled, at aproximal end, to a proximal conductive junction component 202, and, at adistal end, to a distal conductive junction component 204. Proximalcomponent 202 is, in turn, coupled to conductor coil 201 andelectrically couples coil 201 to inductor coil 203; and distal component204 is coupled to electrode 122 and electrically couples coil 203 toelectrode 122. FIG. 2A further illustrates distal conductive junctioncomponent 204 including a proximal protrusion 242 interlocking with core206, a proximal stud 244 on which inductor coil is mounted for coupling,for example, via crimping or laser welding, a distal stud 248 on whichelectrode 122 is mounted for coupling, for example, via crimping orlaser welding, and a shaft 246 extending between proximal stud 244 anddistal stud 248. Proximal conductive junction component 202 is shownincluding a proximal stud 227 on which conductor coil 201 is mounted forcoupling, for example via crimping or laser welding, and a distal stud229 interlocking with core 206 and on which inductor coil 203 is mountedfor coupling, for example, via crimping or laser welding. According topreferred embodiments, core 206 is relatively rigid to transfer torqueand is torsionally coupled to components 202 and 204; core 206 is showninterlocking with proximal component 202, extending within a bore ofstud 229 and through sidewall slots or holes 212, and with distalcomponent 204, extending through hole 214. Suitable materials for core206 include, but are not limited to, relatively hard polyurethanes, suchas 55D or 75D, ceramics and Polyetheretherketones (PEEK®); and core 206may be molded into interlocking engagement with components 202 and 204.

FIG. 3A is a plan view, with a partial section, of an electrode assembly300, according to another embodiment of the present invention. FIG. 3Aillustrates an electrode assembly 300 including an inductance augmenter310 electrically coupled in series between conductor coil 201 andelectrode 122; inductance augmenter 310 includes inductor coil 203 woundabout a non-conductive core 306 and is similar to augmenter 210,previously described. According to the illustrated embodiment, twodistal conductive components 304 and 308 electrically couple electrode122 to inductor coil 203 and torsionally couple electrode 122 to core306; core 306 extends within a bore and sidewall slots or holes 312 tointerlock with component 304 in manner similar to the junction of core306, and core 206, with component 202.

FIG. 3A further illustrates, with dashed lines, component 308 includinga protrusion 382 inserted within a bore 314 of component 304 forcoupling, a distal stud 388 on which electrode 122 is mounted forcoupling, and a shaft 386 extending between protrusion 382 and stud 388;suitable coupling means include, but are not limited to, crimping andlaser welding. Electrode assembly 300 may be incorporated into lead 110in a manner similar to that shown for assembly 200 in FIG. 2B or may beincorporated into a lead of a different type, for example as shown inFIG. 3B.

FIG. 3B is a plan view, with partial sections, of a lead 315 includingelectrode assembly 300, according to some embodiments of the presentinvention. FIG. 3 illustrates assembly 300 as an extendable-retractableassembly wherein the proximal end of conductor is coupled, within aconnector 325, to connector pin 205, as previously described forassembly 200 in lead 110. FIG. 3B further illustrates lead 315 includingan elongate insulation tubing 341 extending distally from connector 325and including a plurality of lumens, two of which are shown: one holdingcoil conductor 201, which is electrically coupled to augmenter 310, aspreviously described in conjunction with FIG. 2A, and another holding acable conductor 331, which is coupled to an electrode surface 328, forexample, via crimping or laser welding.

According to the illustrated embodiment, tubing 341 electricallyisolates conductors 201 and 331 from one another, and from theenvironment external to lead 315, and is joined, for example, byadhesive bonding or welding, to a distal insulator element 345 thatincludes a first lumen, holding an extension of conductor 201 andinductance augmenter 310, and a second lumen, holding an extension ofconductor 331. Insulator element 345 is shown extending beneath anelectrode surface 328 from the junction with tubing 341 to a distal endfrom which electrode 122 protrudes; an outer diameter of element 345, oneither side of electrode surface 328, is approximately equal to an outerdiameter of surface 328, and a distal portion of element 345 houses adistal portion of assembly 300 and a sealing member 318. According to anexemplary embodiment, the outer diameter of element 345 and electrodesurface 328 is about 7 French or between approximately 0.085 inch andapproximately 0.1 inch. FIG. 3B further illustrates lead 315 including acoil electrode 330 extending over the junction between distal insulatorelement 345 and tubing 341 and proximally therefrom; those skilled inthe art will appreciate that coil electrode 330 may serve as adefibrillation electrode and is electrically coupled to anotherconductor extending within another lumen of tubing 341 that cannot beseen in this view. Lead 315 may further include another electrode,disposed proximal to electrode 330, which may serve as a defibrillationelectrode too, thus, tubing 341 may include yet another lumen for aconductor which couples to this other electrode.

FIG. 4 is a section view through an exemplary insulated inductor wire403 from which inductor coil 203 may be formed. FIG. 4 illustratesinsulated inductor wire 403 including a conductive wire 413 over-laidwith an insulating layer or coating 423; according to preferredembodiments of the present invention, wire 413 is formed of asilver-cored MP35N alloy, having up to 40% silver and a diameter rangingfrom approximately 0.001 inch to approximately 0.003 inch, and layer 423is formed of Si polyimide having a thickness of approximately 0.0001inch. The silver-cored MP35N is preferred for its relatively lowresistance combined with an adequate current carrying capacity, butother suitable materials from which wire 413 may be formed include, butare not limited to, tantalum, non-silver-cored MP35N, and copper.Alternate materials for layer 423 include, but are not limited to,fluoropolymers, such as ETFE and PTFE. Insulating layer 423 around wire403 at either end of coil 203 may be mechanically abraded away orthermally ablated away for electrical coupling with the distal andproximal conductive junction components; if laser welding is used toelectrically couple coil 203, the laser energy may simultaneously weldand ablate away layer 203 at the weld.

With reference back to FIG. 3A, exemplary dimensions of coil 203, formedfrom preferred embodiments of wire 403, that provide a sufficientaugmenting inductance to electrode assemblies, for example assemblies200 and 300, will now be described. FIG. 3A illustrates coil 203 beingclose wound and having a length A and an outer diameter B. Theinductance L of a coil is proportional to the number of turns, thelength, and the diameter of the coil according to the followingrelationship: L=(μN²A)/I, where μ is magnetic permeability of the coreabout which the coil is wound, N is the number of turns of the coil, Ais the cross-sectional area of the coil, and I is the length of thecoil. Thus in order to achieve an inductance between approximately 0.3μH and 0.8 μH (that which will effectively augment the inductance ofconductor coils having dimensions in the ranges previously described),length A may range from approximately 0.1 inch to approximately 0.25inch, and outer diameter B may range from approximately 0.02 inch toapproximately 0.04 inch. With reference back to FIGS. 2A and 3A it maybe appreciated that outer diameter D of coil 203 is approximately equalto an outer diameter of coil 201. Although such a configuration (or onein which D is less than the outer diameter of coil 201) is preferred, soas not to increase an outer diameter of the lead into which eitherassembly 200 or 300 are incorporated, the scope of the present inventionis not so limited, and alternate embodiments may include larger diameterinductor coils. Also, it should be noted that although the figuresherein illustrate coil 203 including only a single layer, alternateembodiments of the present invention include inductor coils wound inmultiple layers, as is known to those skilled in the art, which mayboost an inductance of the augmenter toward 4 μH.

FIG. 5A is a plan view, with a partial section, of an inductanceaugmenter assembly 500, according to some embodiments of the presentinvention; and

FIG. 5B is a perspective view of a component of the augmenter assemblyshown in FIG. 5A. FIG. 5A illustrates assembly 500 including inductorcoil 203 electrically coupled in series between a first conductive end502 and a second conductive end 504 and wound about a non-conductivecore 506 that extends between ends 502 and 504. FIG. 5A furtherillustrates first end 502, which generally corresponds to proximalconductive junction component 202 shown in FIGS. 2A-3B, including aproximal stud 527 on which conductor coil 201 may be mounted forelectrical coupling, and second end 504, which generally corresponds todistal conductive junction component 304 shown in FIGS. 3A-B, includinga bore 514 in which protrusion 382 of distal component 308 may beinserted for electrical coupling with electrode 122. According to theillustrated embodiment, first and second ends 502 and 504 each include arelatively flat and longitudinally extending protrusion 529 and 549,respectively, which extend within core 506 to provide torsional couplingwith core 506; each protrusion 529 and 549 further includes slots 520and 540, respectively, for interlocking with core 506 to enhance atensile strength of assembly 500. With reference to FIG. 5B, it may beappreciated that core 506 extends about each protrusion 529, 549,thereby forming a cylindrical profile about which coil 203 extends tobutt up against shoulders 526 and 546 of ends 502 and 504, respectively,for electrical coupling, for example, via laser welding.

FIG. 5A further illustrates core 506 including an optional indentation516, indicated with dashed lines; indentation 516, extending about acircumference of core 506, increases the bending flexibility core 506,without compromising torque transfer, which may be useful forimplementation in some leads, for example, according to embodimentsillustrated by FIG. 3B. FIG. 3B illustrates lead 315 wherein a lumen ofinsulator element 345 carrying a distal portion of electrode assembly300 travels laterally from an offset opening that mates with a lumen oftubing 341, from which assembly 300 extends, to a more central locationto position electrode 122 approximately along a longitudinal centerlineof lead 315. According to the illustrated embodiment, inductanceaugmenter 310 bends along the lumen of element 345; a resilient bendingflexibility of inductance augmenter 310, for example, via indentation516 in core 306, may help to increase the efficiency of torque transferto electrode 122. Various hinge designs, known to those skilled in theart, which facilitate resilient bending, without sacrificing torquetransfer, may alternately be incorporated into inductance augmenterassemblies.

In the foregoing detailed description, the invention has been describedwith reference to specific embodiments. However, it may be appreciatedthat various modifications and changes can be made without departingfrom the scope of the invention as set forth in the appended claims. Forexample, although the present invention has been described in thecontext of screw-in, or active fixation, leads that require torsionalcoupling of inductance augmenters, the invention is not so limited;alternate embodiments of the present invention include passive fixationleads, which are known to those skilled in the art, having inductanceaugmenters which may or may not be torsionally coupled.

1. A medical electrical lead, comprising: an insulated elongateconductor coil extending from a proximal end to a distal end; anelectrode surface electrically coupled to the conductor coil anddisposed distal to the distal end of the conductor coil; an inductanceaugmenter comprising an inductor coil formed of an insulated wireelectrically coupled in series between the electrode surface and theconductor coil, the augmenter further comprising a non-conductive coreabout which the inductor coil is wound; and a distal conductive junctioncomponent coupled to the core and electrically coupling the electrodesurface to the inductor coil.
 2. The lead of claim 1, further comprisinga proximal conductive junction component coupled to the core andelectrically coupling the conductor coil to the inductor coil.
 3. Thelead of claim 1, wherein the distal conductive junction componentincludes a shaft extending between the electrode surface and thecoupling with the core.
 4. The lead of claim 1, further comprising: anelectrode component including the electrode surface; and a shaftextending from a proximal end to a distal end, the shaft electricallycoupled to the distal junction component in proximity to the shaftproximal end and to the electrode component in proximity to the shaftdistal end.
 5. The lead of claim 1, wherein an outer diameter of theconductor coil is approximately equal to an outer diameter of theinductor coil.
 6. The lead of claim 1, wherein an inductance of theinductor coil is between approximately 0.3 μH and approximately 0.8 μH,and a combined inductance of the conductor coil and the inductor coil isgreater than approximately one μH.
 7. The lead of claim 1, wherein thecore has some bending flexibility.
 8. The lead of claim 1, furthercomprising: a second insulated elongate conductor extending alongsidethe conductor coil; a second electrode surface electrically coupled tothe second conductor and disposed proximal to the electrode surface; andan insulator member extending over the inductance augmenter between theelectrode surface and the second electrode surface.
 9. The lead of claim8, wherein an outer diameter of the second electrode surface isapproximately equal to an outer diameter of the insulator member. 10.The lead of claim 8, wherein the second elongate conductor isapproximately coaxial with the conductor coil.
 11. The lead of claim 1,further comprising a helical component including the electrode surface,and wherein the conductor coil, the inductance augmenter, and the distaljunction component all transfer torque to the helical component, thetorque being applied to the conductor coil, in proximity to theconductor coil proximal end.
 12. An electrode assembly for a medicalelectrical lead, comprising: an elongate conductive torque coilextending from a proximal end to a distal end; a helix electrodeelectrically coupled to the torque coil and disposed distal to thedistal end of the torque coil; an inductance augmenter comprising aninductor coil formed of an insulated wire electrically coupled in seriesbetween the electrode and the conductor coil, the augmenter furthercomprising a non-conductive core about which the inductor coil is wound,the core having torque-transfer capacity and being torsionally coupledto the torque coil and the electrode.
 13. The assembly of claim 12,further comprising a proximal conductive junction component torsionallycoupling the core to the torque coil and electrically coupling thetorque coil to the inductor coil.
 14. The lead of claim 12, wherein anouter diameter of the torque coil is approximately equal to an outerdiameter of the inductor coil.
 15. The lead of claim 12, wherein acombined inductance of the torque coil and the inductor coil is greaterthan approximately one μH.
 16. The lead of claim 12, wherein the corehas some bending flexibility.
 17. An inductance augmenter assembly for amedical electrical lead, comprising: a first conductive end including astud to accommodate coupling with a conductor coil of the lead; a secondconductive end; an inductor coil formed of an insulated wireelectrically coupled in series between the first and second conductiveends; and a non-conductive core about which the inductor coil is wound,the core extending between the first and second conductive ends.
 18. Theassembly of claim 17, wherein the core has torque-transfer capacity andis torsionally coupled to the first and second conductive ends.
 19. Theassembly of claim 17, wherein at least one of the first and secondconductive ends includes a relatively flat longitudinal protrusionextending within the core.
 20. The assembly of claim 17, wherein thecore has some bending flexibility.